An investigation began at Sydney Airport today after a blackout crippled air traffic control last night as 20 international and domestic flights circled above managing to avoid catastrophe.

Airservices Australia spokesman Richard Dudley said today the blackout would be investigated both internally and externally.

The blackout caused a computer failure, cutting contact between evening peak-hour services and air traffic controllers, throwing flights into chaos and forcing planes to circle for up to 20 minutes.

Mr Dudley said the actual blackout lasted around two minutes, but rebooting the computer system took another 10 minutes.

"It meant that both air traffic controllers and pilots had to resort to the contingency arrangements," he told Radio National. These included pilot-to-pilot communications and resorting to air traffic controllers at other airports.

Mr Dudley said Airservices regarded the incident extremely seriously and would track down the cause to ensure it did not recur. "We view any sort of loss of service that we provide the airline customers of ours with, with the seriousness it deserves," he said.

"We have had engineers and technicians on site for most of the evening trying to track down the source of the issue. "Under these circumstances, we always will go to an immediate investigation, ... and there will be an external investigation independently undertaken by the Australian Transport Safety Bureau."

Mr Dudley said Airservices believed no near-misses - or breakdowns of separation - between circling aircraft occurred. He did not wish to speculate on the chances of a mid-air collision.

"The bottom line is that there are layers of contingency that we do turn to in such a rare occasion as this," he said. "It is of great concern to us to establish why that
power outage occurred, particularly as we have layers ourselves of uninterruptable power supplies (UPS) underneath.

"For this sort of situation to occur is extraordinarily rare and that's why we want to get to the bottom of it as quickly as possible." The disruption resulted in delays to three outbound international flights and to domestic arrivals and departures, according to airport sources.

But airport officials were unable to give a reason for the blackout. "We had a power supply interruption, the cause of which I'm not sure," said Scott Sloan, a spokesman for Air Services Australia last night.

He said the interruption from about 6.20pm (AEST). A back-up power supply later restored operations.

Ansett spokesman Peter Young said an evening flight from Coolangatta to Sydney had to be turned back during the incident. "A couple of flights had to be held on the ground and there was about a 15 minute delay with one Sydney to Melbourne flight," Mr Young said.

He said the disruptions to Ansett services had ceased within 30 minutes. Sydney Airport spokesman Peter Gibbs said three international departures had been
delayed for between 15 to 20 minutes but arrivals had continued. "We were able to land some aircraft that were still in the air on approach," Mr Gibbs said.

A Qantas spokeswoman said the power failure resulted in minimal disruption to the airline's services. The spokeswoman said that a 6.15pm Brisbane to Sydney flight had been delayed by 30 minutes after the captain anticipated having to circle Sydney Airport on arrival and decided to refuel his aircraft.

But a passenger on the flight said the service had been delayed almost an hour. The captain announced on a public address system that the delay had been caused
by a plane equipment malfunction before saying there were radar problems in Sydney, the passenger said.

"First they told us that an electronic device had to be fixed on the plane delaying the flight for about 20 minutes," she said. "The captain then informed passengers there had been a further delay because the radar was down at Sydney Airport and it wasn't
accepting any aircraft."

Impulse airlines had also experienced delays of between 10 and 20 minutes on three of its flights - one from Canberra, another to Armidale and a jet service to Melbourne.

With just over two months before Sydney hosts the Olympic Games, the technical difficulties come on top of recent problems with the airport's troubled $43 million
baggage handling system at the airport.

They included a 25-minute stoppage of the system on Tuesday and Saturday's 11-hour shutdown which delayed 6000 passengers and left 2000 pieces of luggage behind.

Sydney Airport chief Greg Russell on Wednesday had given a public reassurance that the facility was ready to handle Olympic travellers.

Mr Gibbs said almost $800 million had been spent redeveloping the airport. More work had been done in the past two years of upgrading than had been done in the preceding decade.